Much of today's salt (essentially NaCl) is produced by means of evaporative processes wherein salt is crystallized from brine. The use of high purity brine has various advantages in such a process.
Said brine is typically obtained by solution mining of rock salt deposits. Rock salt, mainly originating from maritime sedimentation, contains alkaline-earth metal (like Ca, Mg and Sr) and potassium salts as the most important impurities. Sulfate, chloride and bromide are typical counter-ions. Together with the sulfate ion, calcium will be present as the rather insoluble CaSO4 (anhydrite) or/and as polyhalite (K2Mg2Ca2(SO4)4.4H2O).
The total amount of calcium and sulfate in rock salt deposits depends on the deposit itself, but, for example, may also vary with the depth at which the salt mined. Calcium is typically present in an amount from 0.5 to 6 gram per kilogram and sulfate from 0.5 to 16 gram per kilogram. Solution mining is a technique with which well soluble salts can be mined at special spots in a deposit. The advantage of this method is that poorly soluble impurities, like anhydrite (CaSO4) and gypsum (CaSO4.2H2O), will remain partly in the cavern being exploited. The resulting brine, however, can be saturated with these undesired impurities. Without any treatment the alkaline (earth) impurities in raw brine, obtained from any of the mentioned sources, will cause severe incrustations in the heating tubes of a vacuum crystallizer of NaCl. Hardly removable calcium sulfate in several appearances will block the tubes and frustrate the heat transfer. Inter alia, contamination of the resulting salt and poor energy efficiency of the process will be the consequence. High purity brine is also of interest for processes wherein salt solutions are used as a raw material, such as in the chemical transformation industry, e.g. the chlorine and chlorate industry. Especially the conversion from mercury and diaphragm technology to the more environmentally acceptable membrane technology triggered the demand for high purity brine. The brine for use in these processes is typically obtained by dissolution of a salt source, which can be rock salt, salt from evaporative processes as described above, and/or solar salt, including lake or sea salt. It is noted that sea salt typically contains less than 0.5 g/l of CaSO4 due to the fact that the CaSO4 is typically present in the form of gypsum with just a limited solubility.
The use of higher purity brine was found to be of interest for this industry because it allows a better energy efficiency as well as the formation of less waste. Also the products resulting from the chemical transformation industry can be of higher quality if brine with high purity is used to make them.
Accordingly, there have been many efforts to improve the quality of brine. A first solution was to use high purity salt, which was dissolved to make such brine. Such high purity salt can be obtained by preventing calcium sulfate from crystallizing in the salt production process by adding specific seeds or by applying a scaling inhibitor. U.S. Pat. No. 3,155,458, for instance, discloses to add starch phosphate to the brine in the evaporative crystallization process. It is said that the starch phosphate enhances the solubility of the CaSO4, and thus prevents the scaling and allows production of salt with high purity and low CaSO4 content.
However, such a process requires the undesired bleed of a CaSO4-rich stream from the crystallization process, and also requires that the brine is essentially bicarbonate-free.
Another solution is to remove impurities from the raw brine by a chemical treatment of said brine. An example of such a treatment is given in the already more than 100 years old Kaiserliches Patentamt DE-115677, wherein hydrated lime is used to precipitated magnesium hydroxide and gypsum from the raw brine.
In addition to, or instead of, these methods, there have also been efforts to increase the purity of the brine by reducing the amount of impurities, such as the above-mentioned anhydrite, gypsum, and polyhalite (and/or their strontium analoques), that dissolve into said brine. This is typically done by adding certain agents to the water that is used in the process, or by mixing such agents with the salt source before adding water (especially for solar salt dissolvers). Hereinafter, such agents are called “retarding agents”.
DD-115341 discloses that brine, particularly for use in processes to make soda ash, with a reduced amount of CaSO4 and MgSO4 can be obtained by adding calcium lignin sulfonate to the water that is used to produce the brine solution. The addition of calcium lignin sulfonate allegedly lowers the solubility of the CaSO4 and MgSO4.
U.S. Pat. No. 2,906,599 discloses to use a group of phosphates, denominated “polyphosphates”, including hexametaphosphates, to reduce the dissolution rate of calcium sulfate (anhydrite), leading to brine with reduced sulfate and calcium ions. At lower concentration (i.e. up to 50 ppm in the brine) hexametaphosphates were found to be the most effective agent, sodium hexametaphosphate being the preferred retarding agent.
Currently, another type of retarding agent is being marketed by Jamestown Chemical Company Inc. under the name (Sulfate Solubility Inhibitor) SSI® 200. According to the material safety data sheet the material contains dodecylbenzene sulfonic acid, sulfuric acid and phosphoric acid.
The present invention alleviates the deficiencies of the prior art by providing for new retarding agent compositions, their use in the process to produce (high purity) brine from a salt source, as well as the use of the so-obtained brine in membrane electrolysis processes.